Hi JLHimes,
First make sure the plugin is activated.
From the WordPress admin dashboard,
To manually compress images:
1.) GOTO Media->Library
2.) There should a column for ‘Compress PNG for WP’ while the library is in ‘list’ mode. (not grid)
3.) Click the link ‘Compress now.’ on the row of the image you wish to manually compress.
OR
2.) Check the box next to the images you wish to manually compress.
3.) Select ‘Bulk Compress via TinyPNG’ from the ‘Bulk Actions’ Dropdown.
4.) Click ‘Apply’
PS. If the image(and its specified resizes) have already been compressed, then attempting to manually recompress will not make another call to TinyPNG’s api thus not counting against your API Key’s monthly limit [free key==500 per month] ).